Brd4 is a double bromodomain-containing protein that binds preferentially to acetylated chromatin. It belongs to the BET (bromodomains and extraterminal) family that includes mammalian Brd2, Brd3, Brd4, Brdt, Drosophila Fsh, yeast Bdf1, Bdf2, and corresponding homologues in other species. Brd4 is essential for cellular growth and has been implicated in cell cycle control, DNA replication, and gene rearrangement found in t(15;19)-associated carcinomas. Recently, Brd4 has been found in several transcription complexes, including the general cofactor Mediator and the P-TEFb elongation factor, and is capable of stimulating HIV-1 transcription in a Tat-independent manner. In addition, Brd4 is used as a cellular adaptor by some animal and human papillomaviruses (HPV) for anchoring viral genomes to mitotic chromosomes. This tethering, mediated by Brd4 interaction with virus-encoded E2 protein, facilitates viral genome segregation during mitosis. Interestingly, Brd4 is also identified in a transcriptional silencing complex assembled by HPV E2 and turns out to be the long sought cellular corepressor that inhibits the expression of HPV-encoded E6 and E7 oncoproteins that antagonize p53 and pRB tumor suppressor activity, respectively. The dual role of Brd4 in gene activation and repression illustrates how a dynamic chromatin-binding adaptor is able to recruit distinct transcriptional regulators to modulate promoter activity through cell cycle progression. Brd4 and BET Family ProteinsBromodomain-containing protein 4 (Brd4) 2 is a member of the BET family that in yeast and animals contains two tandem bromodomains (BDI and BDII) and an extraterminal (ET) domain (1). The bromodomain is a conserved region of ϳ110 amino acids that structurally forms 4 ␣-helices (␣ z , ␣ A , ␣ B , and ␣ C ) and 2 loops, linking ␣ z and ␣ A (ZA loop) and ␣ B and ␣ C (BC loop), capable of binding acetyl-lysine residues in histones and many other proteins (2). In humans, four BET proteins (Brd2, Brd3, Brd4, and Brdt) exhibit similar gene arrangements, domain organizations, and some functional properties. Brd2, formerly named RING3 (really interesting new gene 3) or Fshrg1 (female sterile homeotic related gene 1), is a nuclear serine/threonine kinase possessing chromatin binding activity with preference for acetylated lysine 12 on histone H4 and transcription activity via its association with transcriptional regulators such as E2F1 (3, 4). Brd3 (also called ORFX or Fshrg2) and Brdt (for bromodomain, testis-specific) are less well characterized although mouse Brdt has been reported to induce global chromatin reorganization in an acetylation-dependent manner (5). Brd4, originally named MCAP (mitotic chromosomeassociated protein; Ref. 6) but also called Fshrg4 or Hunk1, is a chromatin binding factor with preference for acetylated Lys-14 on histone H3 and Lys-5/12 on H4 (7). Except for Brdt, which is expressed specifically in testis and ovary, Brd2, Brd3, and Brd4 are widely distributed (8, 9). Interestingly, the chromosomal locations of these Brd genes a...

Triple negative breast cancer (TNBC) is a heterogeneous and clinically aggressive disease for which there is no targeted therapy1-3. BET bromodomain inhibitors, which have shown efficacy in several models of cancer4-6, have not been evaluated in TNBC. These inhibitors displace BET bromodomain proteins such as BRD4 from chromatin by competing with their acetyllysine recognition modules, leading to inhibition of oncogenic transcriptional programs7-9. Here we report the preferential sensitivity of TNBCs to BET bromodomain inhibition in vitro and in vivo, establishing a rationale for clinical investigation and further motivation to understand mechanisms of resistance. In paired cell lines selected for acquired resistance to BET inhibition from previously sensitive TNBCs, we failed to identify gatekeeper mutations, new driver events or drug pump activation. BET-resistant TNBC cells remain dependent on wild-type BRD4, which supports transcription and cell proliferation in a bromodomain-independent manner. Proteomic studies of resistant TNBC identify strong association with MED1 and hyper-phosphorylation of BRD4 attributable to decreased activity of PP2A, identified here as a principal BRD4 serine phosphatase. Together, these studies provide a rationale for BET inhibition in TNBC and present mechanism-based combination strategies to anticipate clinical drug resistance.

In eukaryotes, the core promoter serves as a platform for the assembly of transcription preinitiation complex (PIC) that includes TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH, and RNA polymerase II (pol II), which function collectively to specify the transcription start site. PIC formation usually begins with TFIID binding to the TATA box, initiator, and/or downstream promoter element (DPE) found in most core promoters, followed by the entry of other general transcription factors (GTFs) and pol II through either a sequential assembly or a preassembled pol II holoenzyme pathway. Formation of this promoter-bound complex is sufficient for a basal level of transcription. However, for activator-dependent (or regulated) transcription, general cofactors are often required to transmit regulatory signals between gene-specific activators and the general transcription machinery. Three classes of general cofactors, including TBP-associated factors (TAFs), Mediator, and upstream stimulatory activity (USA)-derived positive cofactors (PC1/PARP-1, PC2, PC3/DNA topoisomerase I, and PC4) and negative cofactor 1 (NC1/HMGB1), normally function independently or in combination to fine-tune the promoter activity in a gene-specific or cell-type-specific manner. In addition, other cofactors, such as TAF1, BTAF1, and negative cofactor 2 (NC2), can also modulate TBP or TFIID binding to the core promoter. In general, these cofactors are capable of repressing basal transcription when activators are absent and stimulating transcription in the presence of activators. Here we review the roles of these cofactors and GTFs, as well as TBP-related factors (TRFs), TAF-containing complexes (TFTC, SAGA, SLIK/SALSA, STAGA, and PRC1) and TAF variants, in pol II-mediated transcription, with emphasis on the events occurring after the chromatin has been remodeled but prior to the formation of the first phosphodiester bond.